Chapter 9: Biomolecules (Cell: Structure and Function)

Macromolecules (biomacromolecules) are the large, polymeric compounds of the acid-insoluble fraction of a living tissue, with molecular weights of 10,000 Da and above, built from many monomer units. Examples: proteins (polymers of amino acids, e.g. haemoglobin, collagen, insulin), nucleic acids (DNA, RNA: polymers of nucleotides) and polysaccharides (starch, cellulose, glycogen, chitin). Lipids also come in this fraction (as membrane vesicles) but are not strictly macromolecules.

The long polypeptide chain (with its helical and other secondary regions) folds upon itself like a hollow woollen ball, giving the protein a definite three-dimensional shape. This tertiary structure creates crevices such as the active site of enzymes and is absolutely necessary for many biological activities of proteins.

(i) Glucose C₆H₁₂O₆ (ii) Ribose C₅H₁₀O₅ (iii) Glycine H₂N–CH₂–COOH (iv) Alanine CH₃–CH(NH₂)–COOH (v) Serine HOCH₂–CH(NH₂)–COOH (vi) Palmitic acid CH₃(CH₂)₁₄COOH (vii) Glycerol HOCH₂–CHOH–CH₂OH (viii) Adenine C₅H₅N₅ (a purine) (ix) Uracil C₄H₄N₂O₂ (a pyrimidine) (x) Cholesterol C₂₇H₄₆O (a sterol). Industries: glucose is made by hydrolysing starch (corn/maize) and sold to food, confectionery and pharmaceutical (IV drips) industries; amino acids such as glutamic acid (MSG) and lysine are made by microbial fermentation and bought by food and feed industries; cholesterol and lecithin are isolated from wool fat and soybean for pharmaceutical and cosmetic buyers.

Therapeutic: insulin (diabetes), interferons (viral infections, cancer), erythropoietin (anaemia), human growth hormone, monoclonal antibodies, vaccines (protein antigens), streptokinase and tissue plasminogen activator (dissolve clots), blood clotting factor VIII (haemophilia), albumin, thrombin. Other uses: collagen and keratin in cosmetics and hair products; proteases and lipases in detergents; rennin in cheese making; papain as a meat tenderiser; gelatin in foods and capsules; enzymes in textile, leather and paper industries.

A triglyceride is made of one molecule of glycerol (trihydroxy propane, HOCH₂–CHOH–CH₂OH) esterified with three fatty acid molecules (R₁, R₂, R₃), which may be the same or different and saturated or unsaturated. Each –OH of glycerol forms an ester bond with the –COOH of a fatty acid, releasing water. Triglycerides are fats (solid) or oils (liquid, lower melting point) depending on their fatty acids.

Yes. Using coloured balls for atoms (black C, white H, red O, blue N, yellow S, purple P) and sticks for bonds, one can build glucose (a six-membered ring), amino acids (an α-carbon with four groups), a peptide bond, glycerol and fatty acids, and parts of a DNA strand. Such models show the 3-D arrangement, bond angles and chirality that formulas on paper cannot.

Alanine is an α-amino acid with R = CH₃: an α-carbon bearing H, an amino group –NH₂, a carboxyl group –COOH and a methyl group –CH₃ (see the figure in the concept notes): CH₃–CH(NH₂)–COOH (C₃H₇NO₂).

Natural gums are secondary metabolites of plants made of complex polysaccharides (heteropolysaccharides of sugars such as arabinose, galactose, rhamnose and glucuronic acid, e.g. gum arabic from Acacia). Fevicol is different: it is a synthetic adhesive, an emulsion of the polymer polyvinyl acetate (PVA), made from petrochemicals, not a biomolecule.

Proteins: biuret test (NaOH + dilute CuSO₄ → violet). Fats and oils: Sudan III (red droplets) or grease-spot test (translucent spot on paper). Amino acids: ninhydrin test (purple/blue on heating). Expected results: fruit juice: sugars, little protein or fat; saliva: proteins present (salivary amylase, mucin), biuret positive; sweat: mostly water, salts and urea, traces of amino acids; urine: normally negative for protein and fat (urea, salts present); protein in urine indicates kidney disease.

Plants of the biosphere produce roughly 10¹¹ tonnes (about 100 billion tonnes) of cellulose per year, the most abundant organic compound. World paper production is about 0.4 billion tonnes a year, which needs about 2–3 tonnes of wood per tonne of paper, i.e. roughly a billion tonnes of plant material, plus much more for fuel wood, timber and cotton. Though a small fraction of annual cellulose, it comes mostly from felling forests, causing loss of vegetation and biodiversity: so recycling paper and planting trees are essential. (Figures are approximate estimates.)

(i) Almost all are proteins (ribozymes are RNA). (ii) They have an active site where the substrate binds, forming an ES complex; they are substrate-specific. (iii) They lower the activation energy and increase reaction rates enormously (carbonic anhydrase about 10 million times). (iv) They are not used up; the enzyme is released unchanged. (v) Activity depends on temperature and pH, with an optimum; they are denatured by heat (thermophile enzymes resist 80–90 °C). (vi) Velocity rises with substrate concentration up to Vmax. (vii) They can be inhibited (e.g. competitive inhibition by malonate). (viii) Many need co-factors (prosthetic groups, co-enzymes, metal ions).